Diastereo- and Enantioselective Photobiocatalytic α-Alkylation of α-Heteroaryl Amines Enabled by Pyridoxal Photochemistry

We report a photobiocatalytic system for the enantio- and diastereoselective α-alkylation of α-heteroaryl amines, a largely underexplored class of synthetically valuable non-native amine donors. Evaluation and directed evolution of thermophilic threonine aldolases, enabled by high-throughput photobiocatalysis, furnished pyridoxal 5'-phosphate (PLP)-dependent enzymes capable of mediating radical C-C bond formation with high levels of stereocontrol. The resulting system enabled the synthesis of diverse chiral aliphatic amines bearing valuable nitrogen heterocycles, including pyridines, pyrimidines, pyrazines, thiazoles, imidazoles, benzothiazoles, and benzimidazoles. Notably, challenging stereochemical elements, such as α-tetrasubstituted stereogenic centers, contiguous trisubstituted stereocenters, and contiguous tri- and tetrasubstituted stereocenters, were readily accessible with our evolved radical C-C coupling enzyme, with excellent diastereo- and enantiocontrol. UV-visible spectroscopy revealed that directed evolution promoted the rapid formation and stabilization of a catalytically essential quinonoid intermediate from non-native amine donors, while mechanistic studies supported an unconventional PLP-enabled photochemical pathway for radical generation from Katritzky salts. Collectively, these results establish pyridoxal radical catalysis as a strategy for expanding chemical space beyond α-amino acids, highlighting its potential to access structurally complex chiral amines with excellent stereocontrol.